The present invention relates in general to apparatus for grinding waste material. It relates in particular to a tub grinder.
Tub grinders are widely used for recovering useable wood material from waste wood products. A tub grinder includes a cylindrical receptacle, or tub, having an open end for receiving the waste wood. The diameter of the receptacle may be large enough that large waste items such as pallets and tree limbs may be placed in it. The tub is located above a generally flat floor and is mounted such that it may rotate relative to the floor about an axis of rotation perpendicular to the floor. Grinding apparatus for grinding the waste wood is located below the floor. The grinding apparatus extends partially through an aperture in the floor so that it may engage material in the tub.
The grinding apparatus may be a hammer-mill including a plurality of grinding elements mounted for rotation in a closed circular path around an axis perpendicular to the axis of rotation of the tub. A portion of the closed circular path extends through the aperture in the floor.
The tub grinder is operated by rotating the grinding elements and the tub. Material in the tub is moved, by the rotation of the tub, continuously across the aperture in the floor and into engagement with the grinding elements. Portions of the waste material are ground into particles by the elements. The particles fall through the aperture where they are further ground, i.e., milled, into smaller particles by the rotating elements. When the milled particles are sufficiently small they pass through one or more screens onto a conveyor which discharges them from the tub grinder.
Waste wood material may contain a number of different metal objects. These metal objects are generally referred to as tramp iron. Tramp iron may include small objects such as nuts, bolts, nails, and screws which may have been used in the construction of some items making up the wood waste. Tramp iron found in waste from yards, construction sites and demolition sites, however, may include relatively large objects, such as hand tools, garden tools, water pipe, electrical conduit, automobile parts, and kitchen appliances.
If tramp iron finds its way into the path of the rotating grinding elements, it may cause rapid wear of the grinding elements or may damage the grinding apparatus such that it will not function.
Apparatus for preventing tramp iron from reaching the grinding elements of a hammer-mill are described in U.S. Pat. Nos. 4,350,307 and 4,896,836. Each patent discloses the use of a magnetized surface for attracting and retaining tramp iron. The magnetized surface is located within a narrow enclosure above the hammer-mill. The narrow enclosure has an open end, or entrance aperture, for introducing waste material into the hammer-mill. In U.S. Patent No. 4,350,307, the magnetized surface is the surface of an inclined flat plate. In U.S. Patent No. 4,896,836, the magnetized surface is the outer surface of a rotating drum. The patents each disclose a stand-alone hammer-mill for grinding or milling wood or agricultural waste which is already in the form of small particles. The waste particles, which may include tramp iron, are gravity-fed into the hammer-mill, first passing over a magnetized surface before they fall into the path of the rotating elements of the hammer-mill. Tramp iron may thus be attracted to and retained by the magnetized surface before it can enter the path of the rotating elements.
In a tub grinder the grinding elements must be exposed directly to bulk waste so that they may break the bulk waste into particles which may subsequently fall into the grinding apparatus where they are further ground or milled. The magnetic extraction systems disclosed in the above references are designed to receive waste material already in particle form. As such, they could not be used to prevent tramp iron from reaching grinding elements of a hammer-mill which is a component of a tub grinder. Further, the magnetic surfaces partially restrict the entrance aperture of the hammer-mill and may result in the hammer-mill being blocked when large tramp iron objects are trapped by the magnetized surface.
Accordingly, it is an object of the present invention to provide apparatus for extracting tramp iron from waste material in a tub grinder.
It is another object of the present invention to provide apparatus for extracting relatively large objects, such as hand tools, garden tools, water pipe, electrical conduit, automobile parts and kitchen appliances, from waste material in a tub grinder.
It is yet another object of the present invention to provide apparatus for attracting and retaining tramp iron from a tub grinder which does not restrict the entrance aperture of a hammer-mill included in the tub grinder.
Additional objects of the invention will be set forth in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combination particularly pointed out in the claims.